state.rs

        data.read_exact(&mut buffer).unwrap();
        self.de = u16::from_le_bytes(buffer);
        let mut buffer = [0x00; 2];
        data.read_exact(&mut buffer).unwrap();
        self.hl = u16::from_le_bytes(buffer);
        let mut buffer = [0x00; 2];
        data.read_exact(&mut buffer).unwrap();
        self.sp = u16::from_le_bytes(buffer);

        let mut buffer = [0x00; 1];
        data.read_exact(&mut buffer).unwrap();
        self.ime = buffer[0] != 0;
        let mut buffer = [0x00; 1];
        data.read_exact(&mut buffer).unwrap();
        self.ie = u8::from_le_bytes(buffer);
        let mut buffer = [0x00; 1];
        data.read_exact(&mut buffer).unwrap();
        self.execution_mode = u8::from_le_bytes(buffer);
        let mut buffer = [0x00; 1];
        data.read_exact(&mut buffer).unwrap();
        self._padding = u8::from_le_bytes(buffer);

        data.read_exact(&mut self.io_registers).unwrap();

        self.ram.read(data);
        self.vram.read(data);
        self.mbc_ram.read(data);
        self.oam.read(data);
        self.hram.read(data);
        self.background_palettes.read(data);
        self.object_palettes.read(data);
    }
}

impl State for BessCore {
    fn from_gb(gb: &mut GameBoy) -> Result<Self, String> {
        let mut core = Self::new(
            Self::bess_model(gb),
            gb.cpu_i().pc(),
            gb.cpu_i().af(),
            gb.cpu_i().bc(),
            gb.cpu_i().de(),
            gb.cpu_i().hl(),
            gb.cpu_i().sp(),
            gb.cpu_i().ime(),
            gb.mmu_i().ie,
            u8::from(gb.cpu().halted()),
            // @TODO: these registers cannot be totally retrieved
            // because of that some audio noise exists
            // The loading of the registers should be done in a much
            // more manual way like SameBoy does here
            // https://github.com/LIJI32/SameBoy/blob/7e6f1f866e89430adaa6be839aecc4a2ccabd69c/Core/save_state.c#L673
            gb.mmu().read_many_unsafe(0xff00, 128).try_into().unwrap(),
        );
        core.ram.fill_buffer(gb.mmu().ram());
        core.vram.fill_buffer(gb.ppu().vram_device());
        core.mbc_ram.fill_buffer(gb.rom_i().ram_data());
        core.oam.fill_buffer(&gb.mmu().read_many(0xfe00, 0x00a0));
        core.hram.fill_buffer(&gb.mmu().read_many(0xff80, 0x007f));
        if gb.is_cgb() {
            core.background_palettes
                .fill_buffer(&gb.ppu_i().palettes_color()[0]);
            core.object_palettes
                .fill_buffer(&gb.ppu_i().palettes_color()[1]);
        }
        Ok(core)
    }

    fn to_gb(&self, gb: &mut GameBoy) -> Result<(), String> {
        gb.cpu().set_pc(self.pc);
        gb.cpu().set_af(self.af);
        gb.cpu().set_bc(self.bc);
        gb.cpu().set_de(self.de);
        gb.cpu().set_hl(self.hl);
        gb.cpu().set_sp(self.sp);

        gb.cpu().set_ime(self.ime);
        gb.mmu().ie = self.ie;

        match self.execution_mode {
            0 => gb.cpu().set_halted(false),
            1 => gb.cpu().set_halted(true),
            2 => gb.cpu().stop(),
            _ => unimplemented!(),
        }

        // @TODO: we need to be careful about this writing and
        // should make this a bit more robust, to handle this
        // special case/situations
        // The registers should be handled in a more manual manner
        // to avoid unwanted side effects
        // https://github.com/LIJI32/SameBoy/blob/7e6f1f866e89430adaa6be839aecc4a2ccabd69c/Core/save_state.c#L1003
        gb.mmu().write_many_unsafe(0xff00, &self.io_registers);

        gb.mmu().set_ram(self.ram.buffer.to_vec());
        gb.ppu().set_vram(&self.vram.buffer);
        gb.rom().set_ram_data(&self.mbc_ram.buffer);
        gb.mmu().write_many(0xfe00, &self.oam.buffer);
        gb.mmu().write_many(0xff80, &self.hram.buffer);

        if gb.is_cgb() {
            // updates the internal palettes for the CGB with the values
            // stored in the BESS state
            gb.ppu().set_palettes_color([
                self.background_palettes.buffer.to_vec().try_into().unwrap(),
                self.object_palettes.buffer.to_vec().try_into().unwrap(),
            ]);

            // updates the speed of the CGB according to the KEY1 register
            let is_double = self.io_registers[0x4d_usize] & 0x80 == 0x80;
            gb.mmu().set_speed(if is_double {
                GameBoySpeed::Double
            } else {
                GameBoySpeed::Normal
            });

            // need to disable DMA transfer to avoid unwanted
            // DMA transfers when loading the state
            gb.dma().set_active(false);
        }

        Ok(())
    }
}

impl Default for BessCore {
    fn default() -> Self {
        Self::new(
            String::from("GD  "),
            0x0000_u16,
            0x0000_u16,
            0x0000_u16,
            0x0000_u16,
            0x0000_u16,
            0x0000_u16,
            false,
            0x00,
            0,
            [0x00; 128],
        )
    }
}

pub struct BessMbrRegister {
    address: u16,
    value: u8,
}

impl BessMbrRegister {
    pub fn new(address: u16, value: u8) -> Self {
        Self { address, value }
    }
}

pub struct BessMbc {
    header: BessBlockHeader,
    registers: Vec<BessMbrRegister>,
}

impl BessMbc {
    pub fn new(registers: Vec<BessMbrRegister>) -> Self {
        Self {
            header: BessBlockHeader::new(
                String::from("MBC "),
                ((size_of::<u8>() + size_of::<u16>()) * registers.len()) as u32,
            ),
            registers,
        }
    }

    pub fn from_data(data: &mut Cursor<Vec<u8>>) -> Self {
        let mut instance = Self::default();
        instance.read(data);
        instance
    }
}

impl Serialize for BessMbc {
    fn write(&mut self, buffer: &mut Cursor<Vec<u8>>) {
        self.header.write(buffer);
        for register in self.registers.iter() {
            buffer.write_all(&register.address.to_le_bytes()).unwrap();
            buffer.write_all(&register.value.to_le_bytes()).unwrap();
        }
    }

    fn read(&mut self, data: &mut Cursor<Vec<u8>>) {
        self.header.read(data);
        for _ in 0..(self.header.size / 3) {
            let mut buffer = [0x00; 2];
            data.read_exact(&mut buffer).unwrap();
            let address = u16::from_le_bytes(buffer);
            let mut buffer = [0x00; 1];
            data.read_exact(&mut buffer).unwrap();
            let value = u8::from_le_bytes(buffer);
            self.registers.push(BessMbrRegister::new(address, value));
        }
    }
}

impl State for BessMbc {
    fn from_gb(gb: &mut GameBoy) -> Result<Self, String> {
        let mut registers = vec![];
        match gb.cartridge().rom_type().mbc_type() {
            MbcType::NoMbc => (),
            MbcType::Mbc1 => {
                registers.push(BessMbrRegister::new(
                    0x0000,
                    if gb.rom().ram_enabled() {
                        0x0a_u8
                    } else {
                        0x00_u8
                    },
                ));
                registers.push(BessMbrRegister::new(
                    0x2000,
                    gb.rom().rom_bank() as u8 & 0x1f,
                ));
                registers.push(BessMbrRegister::new(0x4000, gb.rom().ram_bank()));
                registers.push(BessMbrRegister::new(0x6000, 0x00_u8));
            }
            MbcType::Mbc3 => {
                registers.push(BessMbrRegister::new(
                    0x0000,
                    if gb.rom().ram_enabled() {
                        0x0a_u8
                    } else {
                        0x00_u8
                    },
                ));
                registers.push(BessMbrRegister::new(0x2000, gb.rom().rom_bank() as u8));
                registers.push(BessMbrRegister::new(0x4000, gb.rom().ram_bank()));
            }
            MbcType::Mbc5 => {
                registers.push(BessMbrRegister::new(
                    0x0000,
                    if gb.rom().ram_enabled() {
                        0x0a_u8
                    } else {
                        0x00_u8
                    },
                ));
                registers.push(BessMbrRegister::new(0x2000, gb.rom().rom_bank() as u8));
                registers.push(BessMbrRegister::new(
                    0x3000,
                    (gb.rom().rom_bank() >> 8) as u8 & 0x01,
                ));
                registers.push(BessMbrRegister::new(0x4000, gb.rom().ram_bank()));
            }
            _ => unimplemented!(),
        }

        Ok(Self::new(registers))
    }

    fn to_gb(&self, gb: &mut GameBoy) -> Result<(), String> {
        for register in self.registers.iter() {
            gb.mmu().write(register.address, register.value);
        }
        Ok(())
    }
}

impl Default for BessMbc {
    fn default() -> Self {
        Self::new(vec![])
    }
}

/// Top level manager structure containing the
/// entrypoint static methods for saving and loading
/// [BESS](https://github.com/LIJI32/SameBoy/blob/master/BESS.md) state
/// files and buffers for the Game Boy.
pub struct StateManager;

impl StateManager {
    pub fn save_file(
        file_path: &str,
        gb: &mut GameBoy,
        format: Option<SaveStateFormat>,
    ) -> Result<(), String> {
        let mut file = match File::create(file_path) {
            Ok(file) => file,
            Err(_) => return Err(format!("Failed to open file: {}", file_path)),
        };
        let data = Self::save(gb, format)?;
        file.write_all(&data).unwrap();
        Ok(())
    }

    pub fn save(gb: &mut GameBoy, format: Option<SaveStateFormat>) -> Result<Vec<u8>, String> {
        let mut data = Cursor::new(vec![]);
        match format {
            Some(SaveStateFormat::Bos) | None => {
                let mut state = BosState::from_gb(gb)?;
                state.write(&mut data);
            }
            Some(SaveStateFormat::Bess) => {
                let mut state = BessState::from_gb(gb)?;
                state.write(&mut data);
            }
        }
        Ok(data.into_inner())
    }

    pub fn load_file(file_path: &str, gb: &mut GameBoy) -> Result<(), String> {
        let mut file = match File::open(file_path) {
            Ok(file) => file,
            Err(_) => return Err(format!("Failed to open file: {}", file_path)),
        };
        let mut data = vec![];
        file.read_to_end(&mut data).unwrap();
        Self::load(&data, gb)?;
        Ok(())
    }

    pub fn load(data: &[u8], gb: &mut GameBoy) -> Result<(), String> {
        let data = &mut Cursor::new(data.to_vec());
        if BosState::is_bos(data) {
            let mut state = BosState::default();
            state.read(data);
            state.to_gb(gb)?;
        } else if BessState::is_bess(data) {
            let mut state = BessState::default();
            state.read(data);
            state.to_gb(gb)?;
        } else {
            return Err(String::from("Invalid state file"));
        }
        Ok(())
    }
}